Enforce lazy creation in RouteHandlerContext

[mshustov]

The current implementation of RouteHandlerContext extends context eagerly even if this API is not used. It creates unnecessary pressure on the CPU & Garbage collector. #78957 made core context provider lazy, but other providers are still created immediately.
Context core service should enforce instantiation on demand, which might require changing the IContextProvider interface.
For example, we can make IContextProvider function sync to call it lazily during access time.

core.http.registerRouteHandlerContext('pluginA', (context) => {
  return {
    ping: () => context.core.elasticsearch.legacy.client.callAsInternalUser('ping') as Promise<string>,
  }
});
// buildContext
Object.defineProperty(contextObject, contextId, {
  get() { create lazily }
});

The main reason why IContextProvider is async - some plugins access getCoreServices there

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[pgayvallet]

For example, we can make IContextProvider function sync to call it lazily during access time.

That seems like a pragmatic solution, but that would change the whole IContextProvider API signature, and this interface is also exposed by core's context service, both on client and server side, so we need to check the usages in the codebase.

Another (also breaking) API change to keep the async capabilities would be to change the buildContext / HandlerContextType signature:

kibana/src/core/utils/context.ts

Lines 252 to 255 in 043ef5e

	private async buildContext(
	source: symbol,
	...contextArgs: HandlerParameters<THandler>
	): Promise<HandlerContextType<THandler>> {

return { 
   ...resolvedContext, 
    [contextName]: await provider(exposedContext, ...contextArgs), 
    }; 

could become

return { 
     ...resolvedContext, 
    [contextName]: get: () => await provider(exposedContext, ...contextArgs),  // not the real syntax, would be using `defineProperty `
}; 

then

core.http.registerRouteHandlerContext('pluginA', (context) => {
  return {
    ping: () => context.core.elasticsearch.legacy.client.callAsInternalUser('ping') as Promise<string>,
  }
});

would be

core.http.registerRouteHandlerContext('pluginA', async (context) => {
  return {
    ping: () => (await context.core).elasticsearch.legacy.client.callAsInternalUser('ping') as Promise<string>,
  }
});

This would have a major impact on all IContextProvider usage though, so not sure this is a great option.